This invention relates to a process for the preparation of aqueous dispersions containing phenolic resins and thermoplastics polymeric materials or rubbers and to their use in bonding surfaces together.
The use of liquid structural adhesives is well known. Among such adhesives are those which comprise a mixture of a thermosettable resin, such as a phenolic resole, and a thermoplastic polymeric material, such as poly(vinyl formal) or a nylon polyamide, or a natural or synthetic rubber, this mixture being dissolved or suspended in an organic solvent. These liquid adhesives are used in a variety of structural applications, for example, in the aerospace industry and in the automobile industry. There is, however, a need for structural adhesives of this type in which the organic solvent has been replaced by water, the compositions being aqueous dispersions. The use of such dispersions avoids the dangers of pollution, toxicity, and flammability which are inherent in the use of many organic solvents. There is the further advantage that spillages of aqueous dispersions where water is the continuous phase are easily removed.
However, manufacture of aqueous dispersions from a mixture of a thermosettable resin and a thermoplastics polymeric material or a rubber can often be very difficult on an industrial scale. For example, often it is necessary first to dissolve the thermoplastics polymeric material or rubber in an organic solvent and then to add it to a solution of an aqueous protective colloid and/or surfactant at a controlled rate whilst stirring rapidly and heating to remove the solvent. The resultant dispersion is then blended with the thermosettable resin which is also in the form of an aqueous dispersion. In general, such a process is only successful when very careful control is exercised over the rates at which the various operations are carried out. Also, the pH and the temperature, as well as the quality of the starting materials, are all generally critical for success. Furthermore, special highly efficient (and therefore costly) mixers are needed, as are often vacuum facilities to assist in solvent removal.
It has now been found that stable aqueous dispersions comprising a phenolic resole and a thermoplastics polymeric material or a rubber can be simply and economically prepared without the use of organic solvents or expensive processing equipment by first dissolving the thermoplastics polymeric material or rubber in the phenol, adding suitable surface active agents and/or protective colloids, water, and a source of formaldehyde, and then allowing the phenol and the formaldehyde to react under the conditions of pH, temperature, and agitation used for the manufacture of phenolic resoles.
Reaction between a phenol and formaldehyde in the presence of a thermoplastics polymeric material or a rubber is not inherently novel.
In British Patent Specification Nos. 958,119, 1,178,679, and 1,211,226 there are described adhesive emulsions that are prepared by mixing a natural or synthetic rubber latex with an aqueous solution of a phenol and formaldehyde and allowing these mixtures to react. In British Patent Specification No. 1,263,915 there is described a method of making a rubber cement composition for use in the production of reinforced rubber articles. According to that Specification, a solution or dispersion of a rubber in an organic solvent is mixed with a resin precursor in the same or a different solvent. The resin precursor may be a generator of formaldehyde and an aromatic hydroxy compound such as resorcinol or 1,3-dihydroxynaphthalene. The composition is applied to a reinforcement and dried.
In British Patent Specification No. 1,213,351 the preparation is described of friction linings from a mixture of a friction material and a phenolic resin which had itself been prepared from a mixture which contained 1 to 15% by weight of a nylon polyamide, based on the weight of the phenol. In the example given, sulfuric acid is used as catalyst for a reaction between cresol and formaldehyde in the presence of a nylon.
In an abstract of Japanese Published Application No. 77.06742 (Chemical Abstracts, Vol. 87, Abstract No. 136980y) there are described adhesives prepared from a mixture of phenol, formalin, aqueous sodium hydroxide, and poly(vinyl alcohol). The resultant polymer solution was mixed with a resorcinol-formaldehyde resin before use.
In none of these publications is there a disclosure of the preparation of a stable dispersion by reaction of a phenol, in which there is dissolved a thermoplastic polymeric material or a rubber, with formaldehyde in the presence of water and a surface active agent and/or a protective colloid.